Conventional optical component materials used to transmit high power laser beams, such as glass, fused silica, calcium fluoride and zinc selenide are heated by the laser beam. Typically the material and/or any coatings on it absorb only a very small fraction of the light, but with very high power beams even this small absorption heats the material. This heating contributes to distortion of the laser beam, whether the beam is transmitted or reflected and can stress the component beyond its elastic limit, permanently damaging it. Consequently the manufacturers of these materials endeavor to reduce the absorption of the material.
Often one or more surfaces of these components are coated with thin film optical coatings to enhance or reduce their reflection of light and/or serve other purposes such as protecting the material from chemical attack or scratches. The absorption of light by these coatings must also be minimized if the component is to be exposed to high power light beams and/or if it is important to maximize the optical efficiency of the component.
Many of these multilayer coatings use at least one thin film material that has a low (<1.5) refractive index and at least one that has a higher (>1.6) refractive index. Some coatings may contain more than 50 thin film layers. Generally, the higher refractive index (H) thin film materials absorb more light than the lower refractive index (L) thin film materials.
In order to increase the power handling capability of these coatings, the designers and manufacturers of these coatings try to reduce the absorption (extinction) of light by these coatings by purifying the materials to be coated and optimizing the coating process, which is usually performed in a vacuum chamber. Traditional ellipsometric, spectrophotometric and optical interference measurement techniques can measure the thickness and (the real part of) the refractive index of these films with adequate precision, but are seldom able to accurately measure extinction coefficients less than 10 ppm. Techniques used to measure extinction coefficients less than 10 ppm include laser calorimetry and common path interferometry. In these techniques a light beam, usually from a laser, irradiates and heats the optical component. The resulting temperature increase is measured and used to determine the absorption. The temperature increase can be measured directly (laser calorimetry) or via optical techniques (mirage technique, common path interferometry).
Currently multilayer low absorption coatings must first be deposited in a vacuum chamber, removed from the vacuum chamber and transported to a separate facility where their absorption can be measured. Often the measuring facility is in a different location than the vacuum chamber and this process can take many days. Existing techniques require a light beam powerful enough to heat the coated substrate so the temperature increase can be measured. Some techniques require, in addition, a probe light beam separate from the heating beam.
Manufacturers would like to measure the absorption of each thin film layer in real time as it is being deposited, so that process parameters can be more quickly optimized to reduce the absorption of that and subsequent layers of the same material. The process of minimizing the absorption for just one material can be lengthy as a large number of parameters are involved, including purification procedures for the raw material, preparation procedures to prepare the raw material for use in the vacuum chamber, chamber cleaning procedures, substrate cleaning and preparation procedures, substrate temperature during and immediately after deposition, chamber pressure, chamber atmosphere, deposition rate, post deposition bake-out procedures and auxiliary deposition parameters (such as ion assisted coating).
Therefore, there is a need for a method and apparatus that can measure the absorption in a low absorption thin film in real time as it is being deposited in a vacuum chamber. The results of these measurements would aid coaters in developing procedures and materials that further reduce the absorption of their coatings. This will result in lower loss coatings that will increase the power handling capability of coated optical components in high power laser applications and increase the finesse or throughput of coated optical components in sensitive instruments, such as interferometers, ring-down cavities or optical systems for microlithography and nanolithography.